The White House is carving out a role for President Barack Obama in this fall's midterm elections, which he will try to pose as a choice between the parties' economic visions while targeting appeals to African-Americans, young voters and others in his coalition.

The driving theory in the White House is that this election, like every one since the 2007 recession, is foremost about the economy. Mr. Obama already has been drawing contrasts between his economic program and that of Republicans, and he is preparing additional...